Error 4121 typically would indicate a corrupt index file. This would not be caused by changing the 'next invoice batch' number to 1.

Did you have any open batches before you did this change? Was the batch status report printed and purged? I had assumed you had done both of these things prior to resetting the batch number.

Does your data pass an integrity check?

When you open the batch screen to show all the open batches, note the batch numbers. Then use Windows Explorer to go to the data directory and take a look. For every batch number you have, you should see 2 data files QIBxxxxI and QIBxxxxR (where xxxx is the batch number). If you have any other data files of the type QIBxxxxI (or R) but don't actually have that open batch number, then these files do not belong. Alternately, you may have a correct batch control file, thus the reason for not being able to add a new batch.

My suggestion is as follows:
1) Make a backup copy of your QI data
2) Open your Invoice program, and Archive all batches possible. Those batches that cannot be archived (due to error messages) will be lost.
3) Close the program, and go back to Explorer and the data directory.
4) Rename (or remove or delete) all data files that start with QIB. This would encompass all open batches and the batch control files.
5) Open Invoices again. This will create brand new batch control files. Also make sure the 'next batch number' is 1.
6) If you archived any batches, open the Archive batches function and Restore your batches back.

Regards,
Softrak Support